--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libstdc++-v3/testsuite/26_numerics/bit/bit.pow.two/bit_floor.cc	Mon May 25 07:50:57 2020 +0900
@@ -0,0 +1,100 @@
+// Copyright (C) 2018-2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+#include <bit>
+#include <limits>
+
+template<typename UInt>
+constexpr auto
+test(UInt x)
+-> decltype(std::bit_floor(x))
+{
+  static_assert( noexcept(std::bit_floor(x)) );
+
+  static_assert( std::bit_floor(UInt(0)) == 0 );
+  static_assert( std::bit_floor(UInt(1)) == 1 );
+  static_assert( std::bit_floor(UInt(2)) == 2 );
+  static_assert( std::bit_floor(UInt(3)) == 2 );
+  static_assert( std::bit_floor(UInt(4)) == 4 );
+  static_assert( std::bit_floor(UInt(0x11)) == 0x10 );
+  static_assert( std::bit_floor(UInt(0x20)) == 0x20 );
+
+  if constexpr (std::numeric_limits<UInt>::digits > 8)
+  {
+    static_assert( std::bit_floor(UInt(0x201)) == 0x200 );
+    static_assert( std::bit_floor(UInt(0x8ff)) == 0x800 );
+    static_assert( std::bit_floor(UInt(0x1000)) == 0x1000 );
+  }
+
+  if constexpr (std::numeric_limits<UInt>::digits > 32)
+  {
+    static_assert( std::bit_floor(UInt(0xabcdef)) == 0x800000 );
+    static_assert( std::bit_floor(UInt(0x1000000)) == 0x1000000 );
+    static_assert( std::bit_floor(UInt(0x1000001)) == 0x1000000 );
+  }
+
+  if constexpr (std::numeric_limits<UInt>::digits > 64)
+  {
+    static_assert( std::bit_floor(UInt(1) << 64) == (UInt(1) << 64) );
+    static_assert( std::bit_floor(UInt(3) << 64) == (UInt(2) << 64) );
+  }
+
+  return true;
+}
+
+static_assert( test( (unsigned char)0 ) );
+static_assert( test( (unsigned short)0 ) );
+static_assert( test( (unsigned int)0 ) );
+static_assert( test( (unsigned long)0 ) );
+static_assert( test( (unsigned long long)0 ) );
+
+// std::bit_floor(T) shall not participate in overload resolution
+// unless T is an unsigned integer type.
+struct X { constexpr bool did_not_match() { return true; } };
+constexpr X test(...) { return X{}; }
+static_assert( test( (bool)0 ).did_not_match() );
+static_assert( test( (char)0 ).did_not_match() );
+static_assert( test( (int)0 ).did_not_match() );
+static_assert( test( (char16_t)0 ).did_not_match() );
+static_assert( test( (float)0 ).did_not_match() );
+static_assert( test( (void*)0 ).did_not_match() );
+static_assert( test( X{} ).did_not_match() );
+enum E : unsigned { e };
+static_assert( test( e ).did_not_match() );
+
+#if !defined(__STRICT_ANSI__) && defined _GLIBCXX_USE_INT128
+static_assert( test( (unsigned __int128)0 ) );
+static_assert( test( (__int128)0 ).did_not_match() );
+#endif
+#if defined(__GLIBCXX_TYPE_INT_N_0)
+static_assert( test( (unsigned __GLIBCXX_TYPE_INT_N_0)0 ) );
+static_assert( test( (__GLIBCXX_TYPE_INT_N_0)0 ).did_not_match() );
+#endif
+#if defined(__GLIBCXX_TYPE_INT_N_1)
+static_assert( test( (unsigned __GLIBCXX_TYPE_INT_N_1)0 ) );
+static_assert( test( (__GLIBCXX_TYPE_INT_N_1)0 ).did_not_match() );
+#endif
+#if defined(__GLIBCXX_TYPE_INT_N_2)
+static_assert( test( (unsigned __GLIBCXX_TYPE_INT_N_2)0 ) );
+static_assert( test( (__GLIBCXX_TYPE_INT_N_2)0 ).did_not_match() );
+#endif
+
+#include <cstddef>
+static_assert( test( (std::byte)0 ).did_not_match() );